1. Field of the Invention
The present invention relates to a semiconductor laser device which oscillates in a fundamental transverse mode.
2. Description of the Related Art
Practical optical output power of high-quality narrow-stripe semiconductor laser devices oscillating in a fundamental transverse mode is increasing year by year. Recently, the practical optical output power of fundamental-transverse-mode semiconductor laser devices is remarkably increasing. In particular, the practical optical output power of 0.98 xcexcm band fundamental-transverse-mode semiconductor laser devices which are used for exciting an Er doped optical fiber amplifier is 250 mW or more. In addition, the practical optical output power of 0.78 xcexcm band and 0.65 xcexcm band fundamental-transverse-mode semiconductor laser devices are also increasing, where the 0.78 xcexcm band and 0.65 xcexcm band fundamental-transverse-mode semiconductor laser devices are used in recordable CDs (compact disks) and recordable DVDs (compact disks), respectively. The increase in the practical optical output power is also important in various applications in the fields of image recording. Further, semiconductor laser devices having higher output power are required in digital drive printers, such as laser thermal printers, using sensitized material having relatively low sensitivity.
In the above applications, stability of the fundamental transverse mode is required as well as the high output power and high reliability of the semiconductor laser devices. However, the output power of the fundamental-transverse-mode semiconductor laser devices has an upper limit which is determined by the two factors.
The first factor is decrease in the reliability due to facet degradation caused by high optical density at the light-exit end facet. The facet degradation can be greatly reduced by high-quality end-facet coating or use of an unabsorbent end-facet window structure. The high-quality end-facet coating is disclosed in IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, p. 832 (1999). The use of the unabsorbent end-facet window structure is disclosed in IEEE Journal of Quantum Electronics, Vol. QE-15, p. 775 (1979), IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, p. 817 (1999), Japanese Journal of Applied Physics, Vol. 30, p. L904 (1991), and Electronics Letters, Vol. 34, p. 1663 (1998).
The second factor is stability of the fundamental mode. In high-power semiconductor laser devices having output power in excess of 200 mW, the stripe width W and the equivalent-refractive-index difference xcex94n in the direction parallel to the active layer (i.e., a difference in the equivalent refractive index between the area of the active region under the stripe and the other area) in the index-guided structure are designed so that higher transverse modes are cut off. FIG. 5 shows a primary mode cut-off condition in the case where the oscillation wavelength is 1,060 nm. In the hatched area under the curve indicated in FIG. 5, the semiconductor laser device can stably oscillate in the fundamental mode. For example, when the stripe width W is 2.5 micrometers, and the equivalent-refractive-index difference xcex94n is 4xc3x9710xe2x88x923, the semiconductor laser device can stably oscillate in the fundamental transverse mode. However, when the output power of the actual semiconductor laser devices is increased to hundreds of milliwatts, the optical output-current characteristic deviates from a straight line, and the radiation pattern is deformed. This is the upper limit of the stable fundamental transverse mode. Since the carrier density of the active layers of semiconductor laser devices is about 1018 cmxe2x88x923, the refractive index is decreases by about 2xc3x9710xe2x88x923 due to the plasma effect, and spatial hole burning of the carrier density is caused by the increase in the optical output power. Accordingly, the refractive index irregularly varies in the direction of the extent of the active layer. Therefore, although the higher modes do not occur, the laser beam is shifted in the horizontal direction, i.e., a so-called beam steering occurs. Thus, the fundamental transverse mode becomes unstable.
In addition, high-output-power semiconductor laser devices usually include a so-called SCH (separate confinement heterostructure) having a quantum well. As reported in Applied Physics Letters, Vol. 75, p. 1839 (1999), it is known that the reliability of the semiconductor laser devices in a high-output-power operation increases when the thickness of the optical waveguide layer is increased, since the optical density at the light-exit end face is reduced by the increase in the thickness of the optical waveguide layer. However, when the thickness of the optical waveguide layer is increased, the amount of light penetrated into the cladding layer is decreased. Therefore, it is not easy to realize a difference in the refractive index from the active layer. Further, the present inventors have found that the transverse modes become unstable when the thickness of the optical waveguide layer is great. Since the distance between the active layer and the current confinement layer is increased by the thickness of the optical waveguide layer, carriers widely spread in the direction parallel to the active layer when the thickness of the optical waveguide layer is great. Thus, the transverse modes become unstable.
An object of the present invention is to provide a semiconductor laser device which can stably oscillate in a fundamental transverse mode even when the output power is high.
According to the present invention, there is provided a semiconductor laser device having an index-guided structure and oscillating in a fundamental mode, comprising: a lower cladding layer; a lower optical waveguide layer formed above the lower cladding layer; a quantum well layer formed above the lower optical waveguide layer; an upper optical waveguide layer formed above the quantum well layer; and a current confinement structure formed above the upper optical waveguide layer. The upper optical waveguide layer has a first thickness smaller than a second thickness of the lower optical waveguide layer. Further, additional semiconductor layers may be formed between the above layers.
In other words, the semiconductor laser device according to the present invention comprises an SCH structure in which a quantum well layer is sandwiched between optical waveguide layers, and the quantum well layer is not located at the center of the SCH structure. That is, the SCH structure is not symmetric about the quantum well layer, which is located relatively near to the current confinement layer. According to this arrangement of the SCH structure, the current injected into the semiconductor laser device does not widely spread, and therefore the semiconductor laser device according to the present invention can maintain a stable oscillation in a fundamental transverse mode even when the output power is high.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (viii).
(i) Barrier layers may be formed between the quantum well layer and the upper and lower optical waveguide layers.
(ii) The sum of the first and second thicknesses may be 0.5 micrometers or greater. In this case, the semiconductor laser device can oscillate with higher output power.
(iii) The distance between the bottom of the current confinement structure and the upper surface of the quantum well layer may be smaller than 0.25 micrometers. The bottom of the current confinement structure determines the stripe width of the current confinement structure. When the index-guided structure is realized by a ridge structure, the bottom of the current confinement structure is the bottom of the ridge portion. When the index-guided structure is realized by an internal stripe structure, the bottom of the current confinement structure is the bottom of a stripe groove in a current confinement layer. When the semiconductor laser device has the additional feature (iii), the spread of the current injected into the semiconductor laser device can be limited to a small extent. Therefore, the maximum optical output power obtained by a stable oscillation in the fundamental transverse mode is increased.
(iv) In the semiconductor laser device having the additional feature (iii), the bottom of the current confinement structure may be arranged on the upper surface of the quantum well layer. Generally, the current confinement structure is formed in a cladding layer. In the semiconductor laser device according to the present invention, a portion of the upper cladding layer may be formed between the upper optical waveguide layer and the bottom of the current confinement structure. However, it is preferable that the bottom of the current confinement structure is arranged on the upper surface of the upper optical waveguide layer, i.e., no layer intervenes between the upper optical waveguide layer and the bottom of the current confinement structure.
(v) The lower optical waveguide layer, the quantum well layer, and the upper optical waveguide layer may be made of an aluminum-free semiconductor material. In this case, the respective layers are less prone to oxidation during formation of the layers. Therefore, the reliability of the semiconductor laser device is increased.
(vi) In the semiconductor laser device having the additional feature (v), the at least one of the lower cladding layer and the upper cladding layer may be made of a semiconductor material containing aluminum. In this case, the energy gap of the cladding layer is increased, and the leakage of electrons can be prevented. Therefore, temperature characteristics are improved.
(vii) The index-guided structure may be an internal stripe type or a ridge waveguide type.
(viii) The index-guided structure may have a stripe width of 4 micrometers or smaller.
The semiconductor laser device according to the present invention oscillates in a fundamental transverse mode. The values of the stripe width and the equivalent-refractive-index difference are selected so that the condition for the fundamental-transverse-mode oscillation is satisfied, and the higher modes can be cut off in the respective oscillation wavelength bands. For example, when the oscillation wavelength is 1,060 nm, the values of the stripe width and the equivalent-refractive-index difference are in the hatched area under the curve indicated in FIG. 5. In particular, the frequently used values of the stripe width and the equivalent-refractive-index difference are in vicinities of 2.5 micrometers and 4xc3x9710xe2x88x923, respectively.
When the index-guided structure is realized by an internal stripe structure, the equivalent-refractive-index difference is a difference in the equivalent refractive index in propagation modes in the thickness direction, between portions of the active region under the current confinement layer and the other portion of the active region under the stripe groove, which is formed in the current confinement layer, and filled with a portion of the cladding layer. When the index-guided structure is realized by a ridge structure, the equivalent-refractive-index difference is a difference in the equivalent refractive index in propagation modes in the thickness direction, between portions of the active region under the ridge portion and the other portion of the active region.